Angle correction for small animal tumor imaging with spatial frequency domain imaging (SFDI) Yanyu Zhao,1 Syeda Tabassum,2 Shaheer Piracha,1 Mohan Sobhana Nandhu,3 Mariano Viapiano,3 and Darren Roblyer1,* 1
2
Boston University, Department of Biomedical Engineering, 44 Cummington Mall, Boston, Massachusetts 02215, USA Boston University, Department of Electrical & Computer Engineering, 8 Saint Mary’s Street, Boston, Massachusetts 02215, USA 3 Brigham and Women's Hospital, Harvard Medical School, 4 Blackfan Circle, Boston, Massachusetts 02115, USA *
[email protected]
Abstract: Spatial frequency domain imaging (SFDI) is a widefield imaging technique that allows for the quantitative extraction of tissue optical properties. SFDI is currently being explored for small animal tumor imaging, but severe imaging artifacts occur for highly curved surfaces (e.g. the tumor edge). We propose a modified Lambertian angle correction, adapted from the Minnaert correction method for satellite imagery, to account for tissue surface angles up to 75°. The method was tested in a hemisphere phantom study as well as a small animal tumor model. The proposed method reduced µa and µs` extraction errors by an average of 64% and 16% respectively compared to performing no angle correction, and provided more physiologically agreeable optical property and chromophore values on tumors. ©2016 Optical Society of America OCIS codes: (170.3880) Medical and biological imaging; (170.5280) Photon migration; (290.1990) Diffusion; (170.0110) Imaging systems.
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1. Introduction Spatial Frequency Domain Imaging (SFDI) is a widefield imaging technique that can be used to quantify optical properties (absorption and reduced scattering) of diffusive media including biological tissue [1,2]. When optical properties at multiple wavelengths are measured, tissue chromophore concentrations can be extracted to help identify disease states, therapy response, and tissue metabolic function. SFDI is being explored for a number of preclinical and clinical applications, including skin flap viability, burn wound healing, and subsurface tomography [3–16]. Recently, our group and others have begun to investigate SFDI as a new tool to understand the in vivo tumor state in small animal oncology models. The application of SFDI to small animal imaging is complicated by the relatively small feature size of the tissues of interest, and the relative high surface curvature of subcutaneous tumors, which may protrude near-orthogonal to surrounding tissue for some models. Observationally, tumor edges, and other surfaces with a high surface normal angle in reference to the camera line of sight, suffer from extreme edge artifacts in SFDI, leading to physiologically implausible optical properties and chromophore concentrations in these regions. Typically, these artifacts manifest as underestimates of diffuse reflectance at low spatial frequencies. One potential method to mitigate these artifacts is to eliminate these steep surfaces from the data using a threshold method based on tissue angle. Unfortunately, this has the effect of censoring large parts of the tumor, which may be unacceptable for many applications. Gioux et al. reported a Lambertian correction method for SFDI which could mitigate edge imaging artifacts for surface angles up to 40° [17]. For this method, a cosine divisor term was applied to SFDI data after image demodulation to increase diffuse reflectance values for surfaces at tilt angles. This method was shown to improve optical property extraction on tissue-simulating phantoms and human hand data, although corrections were limited to angles less than 40°. We expand on this work by applying the so-called Minnaert Correction, which was first proposed for lunar photometry and later developed to angle-correct satellite imagery from the effects of solar illumination angles and relative terrain angles [18,19]. In the context of SFDI measurements, we refer to this correction as the Modified Lambertian Correction (MLC). The MLC is a parameter optimization method that adds an additional correction factor to the Lambertian correction by empirically accounting for inter-object diffuse
#260851 (C) 2016 OSA
Received 9 Mar 2016; revised 12 May 2016; accepted 16 May 2016; published 24 May 2016 1 June 2016 | Vol. 7, No. 6 | DOI:10.1364/BOE.7.002373 | BIOMEDICAL OPTICS EXPRESS 2374
reflectance (e.g. light reflected off surrounding normal tissue onto the tumor), as well as other possible contributions to inaccurate diffuse reflectance values, especially near the tumor edge. To validate the MLC method, SFDI measurements were taken on hemispheric tissuesimulating optical phantoms with a range of optical properties and different sizes, fabricated to mimic the geometry of subcutaneous xenografted tumors. The MLC method was compared against non-angle and the standard Lambertian correction for both lower angles (
